One type of drill used in forming a borehole in the earth is a roller cone drill bit. A typical roller cone bit comprises a bit body with an upper end adapted for connection to a drill string. Depending from the lower portion of the bit body are a plurality of support arms, typically three, each with a spindle protruding radially inward and downward with respect to a projected rotational axis of the bit body. A cutter cone is generally mounted on each spindle and supported rotatably on bearings acting between the spindle and the inside of a spindle-receiving cavity in each cutter cone. One or more fluid nozzles are often formed on the underside of the bit body. The nozzles are typically positioned to direct drilling fluid passing downwardly from the drill string toward the bottom of the borehole being drilled. Drilling fluid washes away material removed from the bottom of the borehole and cleanses the cutter cones, carrying the cuttings and other debris radially outward and then upward within an annulus defined between the drill bit and the wall of the borehole.
Protection of bearings which allow rotation of the cutter cones can lengthen the useful service life of a drill bit. Once drilling debris is allowed to infiltrate between bearing surfaces of the cutter cone and spindle, failure of the drill bit will follow shortly. Various mechanisms have been employed to help keep debris from entering between the bearing surfaces. A typical approach is to place an elastomeric seal across a gap between the bearing surfaces of each cutter cone and its respective spindle support arm. However, once the seal fails, it is not long before drilling debris contaminates the bearing surfaces via the gap between the cutter cone and its respective spindle. Thus, it is important that each seal also be protected against wear caused by debris in the borehole.
Various approaches have previously been employed to protect seals in drill bits from debris in the well bore. One approach is to provide hardfacing and wear buttons on opposite sides of the gap between each spindle support arm and the respective cutter cone where the gap opens to the exterior of the drill bit and is exposed to debris-carrying well fluid. Hardfacing and wear buttons slow erosion of the metal adjacent to the gap, and thus prolonging the time before the respective seal is exposed to borehole debris. Another approach is to construct inner-fitting parts of the cutter cone and the spindle support arm to produce a tortuous path within each gap leading to the respective seal which is difficult for debris to follow. Examples of drill bits with seal protection features are disclosed in U.S. Pat. Nos. 4,613,004 entitled Earth Boring Bit with Labyrinth Seal Protector, and 4,037,673 entitled Roller Cutter Drill Bit.
An example of the first approach is used in a conventional tri-cone drill bit wherein the base of each cutter cone at the juncture with the respective spindle and support arm is defined at least in part by a substantially frustoconical surface, termed the cutter cone gage surface. This cutter cone gage surface is slanted in an opposite direction as compared to the conical surface of the shell or nose of the respective cutter cone and often includes hardfacing, a plurality of hard metal buttons or surface compacts. The latter are designed to reduce the wear of the frustoconical portion of the cutter cone gage surface at one side of the gap. On the other side of the gap, the lower portion of the respective support arm is protected by hardfacing material. For definitional purposes, the lower portion of the support arm located on the exterior of the associated drill bit below the nozzles is often referred to as a shirttail surface or simply shirttail. More specifically, in referring to prior art drill bits, radially outward of the juncture of each spindle with its respective support arm, and toward the exterior of the drill bit, the lower edge or extreme lower portion of the shirttail is referred to as the tip of the shirttail or shirttail tip.
During drilling with rotary cone drill bits of the foregoing character, debris often passes between the cutter cone gage surface and the wall of the borehole generally within the area where the gap opens to the borehole annulus. As a result, the edge of the shirttail tip of each support arm which leads in the direction of rotation of the drill bit during drilling, i.e., the leading edge, can become eroded. As this erosion progresses, the hardfacing covering the respective shirttail tip eventually erodes or chips off. This erosion exposes underlying softer metal to increased erosion and thereby shortens the path that debris may take through the gap to the respective seal and ultimately exposes the respective seal to borehole debris causing seal failure and ultimately bearing failure.
Generally, the shirttail tip of a conventional support arm is relatively thin and does not allow application of hardfacing in sufficient quantities to provide adequate protection from erosion. Also, tungsten carbide inserts must be located away from the thin shirttail tip to accommodate clearance and material strength requirements for press fit installation. Depending upon specific downhole drilling conditions and design geometry of the respective rotary cone drill bit, the thin shirttail tip of the respective support arms may be a weak point leading to failure of the associated drill bit.